Toner concentration regulating apparatus

ABSTRACT

An apparatus in which the concentration of toner powder in a particulate mixture is regulated. The particulate mixture is utilized in a development system for producing a viewable toner pattern on an image bearing member.

United States Patent [191 Knapp et al.

[111 3,801,196 Apr. 2, 1974 1 1 TONER CONCENTRATION REGULATING APPARATUS[75] lnventors: Lowell W. Knapp, Victor; Robert P.

Rebres, Fairport, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: Apr. 20, 1973 [211 App]. No.: 353,029

[52] U.S. Cl 355/3, 355/4, 118/637, 117/17.5 51 Int. Cl (i03g 1519s [58]Field of Search 355/3, 4; 118/637, 7; 117/175 [56] References CitedUNITED STATES PATENTS 3,348,522 10/1967 Donohue ..ll8/7 Hawk 118/7Davidson ct al. I 18/7 OTHER PUBLICATIONS IBM Technical DisclosureBulletin, Boggs et al., Vol. 15, No. 4, 9-72.

Primary Examiner-Robert P. Greiner Attorney, Agent, or Firm-James J.Ralabate; Henry Fleischer; Clarence A. Green [5 7] ABSTRACT An apparatusin which the concentration of toner powder in a particulate mixture isregulated. The particulate mixture is utilized in a development systemfor producing a viewable toner pattern on an image bearing member.

4 Claims, 3 Drawing Figures PATENTEDAPR 2 I974 SHLU 1 [IF 2 PATENTEDAPR2 m4 3801196 SHEET 2 [IF 2 TONER CONCENTRATION REGULATING APPARATUSBACKGROUND OF THE INVENTION This invention relates generally to amulti-color electrophotographic printing machine, and more particularlyconcerns an apparatus for regulating the concentration of tonerparticleswithin the developer mix used in the development systemthereof.

Toner particle concentration in the developer mix directly effects thedevelopability of the multi-color electrophotographic printing machine.The concentration of toner particles within the developer mix, i.e., thepercentage of toner particles relative to carrier, relates directly tothe characteristics of the developed image. For example, the density ofthe image will be affected by the toner particle concentration.

Various types of systems have been developed which add toner particlesto the developer mix. However, most of these systems are directedprimarily to black and white printing machines rather than multi-colorprinting machines. A typical system used in a black and white printingmachine is disclosed in US. Pat. No. 3,399,652 issued to Gawron in 1968.This patent discloses a rotating disc disposed in the developer mix. Thedisc is electrically biased to attract toner particles from the mix. Alight beam is reflected from the surface of the disc onto aphotoelectric unit. The intensity of light striking the photoelectricunit is an indication of the toner particle concentration within thedeveloper mix. However, the foregoing system is not mounted on thephotoconductive member and, hence, does not undergo the normalprocessing steps thereof. The apparatus disclosed in Gawron appears tobe located in a developer mix sump external to the photoconductivemember.

One system adapted for use in a multi-color electrophotographic printingmachine is described in copending application Ser. No. 213,056, filed in1971. As disclosed therein, the apparatus includes a transparentelectrode mounted on the photoconductive member and adapted to attractelectrostatically toner particles thereto. A light source generates abeam of light rays which are transmitted from the interior of thephotoconductive drum through the transparent electrode onto aphotosensor. The photosensor develops an electrical signal indicative ofthe density of toner particles adhering to the transparent electrode. Inthe foregoing system, light rays pass through the transparent electroderather than being reflected therefrom. This introduces the addedcomplexity of transmitting a light beam to the interior of thephotoconductive drum so that it may pass through the transparentelectrode.

Accordingly, it is a primary object of the present invention to improvethe apparatus used to regulate toner particle concentration within adeveloper mix.

SUMMARY OF THE INVENTION Briefly stated, and in accordance with thepresent invention, there is provided an apparatus for regulating tonerpowder concentration within a particulate mixture utilized in adevelopment system arranged to deposit toner powder on an image bearingmember.

'Pursuant to the present invention, reflecting means, mounted on theimage bearing member, is biased electrically to a potential ofsufficient magnitude to attract the toner powder thereto. Thus as thedevelopment system deposits toner powder on the image bearing member,toner powder is attracted to the reflecting means. Means are providedfor generating a beam of light rays. The reflecting means is arranged tobe in a light receiving relationship with the beam of light rays afterthe toner powder is attracted thereto. In addition, detecting meanssense the intensity of light rays transmitted from the reflecting meanshaving the toner powder adhering thereto. Preferably, the detectingmeans is adapted to produce an electrical output signal indicative ofthe toner powder density deposited on the reflecting means.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of thepresent invention will become apparent upon reading the followingdetailed description and upon reference to the drawings, in which:

FIG. 1 is a schematic perspective view of a multicolorelectrophotographic printing machine having the features of the presentinvention therein;

FIG. 2 is a fragmentary view of the regulating apparatus of the presentinvention; and

FIG. 3 is a perspective view of the reflecting arrangement utilized inthe FIG. 2 apparatus.

While the present invention will be described in connection with apreferred embodiment thereof, it will be understood that it is notintended to limit the invention to that embodiment. On the contrary, itis intended to cover all alternatives, modifications and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE INVENTION For a general understanding of thedisclosed electrophotographic printing machine in which the presentinvention may be incorporated, continued reference is had to thedrawings wherein like reference numerals have been used throughout todesignate like elements. FIG. 1 schematically illustrates the variouscomponents of the multi-color printing machine. Although the tonerconcentration regulating apparatus of the present invention isparticularly well adapted for use in a multicolor electrophotographicprinting machine, it should become evident from the following discussionthat it is equally well suited for use in a wide variety of printingmachines and is not necessarily limited in its use to the particularembodiment shown herein.

As shown in FIG. 1, the multi-color electrophotographic printing machineutilizes a drum 10 mounted rotatably within the machine frame (notshown). Photoconductive surface 12 is mounted on the exteriorcircumferential surface of drum 10. One type of suitable photoconductivematerial is disclosed in U. S. Pat. No. 3,655,377 issued to Sechak in1972. A series of processing stations are disposed about drum 10 suchthat as it rotates in the direction of arrow 14, it passes sequentiallytherethrough. A drive motor rotates drum 10 at a predetermined speedrelative to the other components of the printing machine. The machinelogic coordinates the various operations with one another to produce theproper sequence of events at the corresponding processing stations.

Initially, as drum 10 rotates in the direction of arrow 14,photoconductive surface 12 passes through charging station A. Chargingstation A has positioned thereat a corona generating device, indicatedgenerally at 16. Corona generating device 16 extends in a generallytransverse direction across photoconductive surface 12 and produces aspray of ions for charging photoconductive surface 12 to a substantiallyuniform potential. U. S. Pat. No. 2,778,946 issued to Mayo in 1957describes a suitable corona generating device of the type utilizedherein.

Thereafter, drum rotates charged photoconductive surface 12 to exposurestation B. At exposure station B, charged photoconductive surface 12 isexposed to a color filtered light image of the original document.Exposure station B includes thereat a moving lens system, generallydesignated by the reference numeral 18, and a color filter mechanismshown generally at 20. As shown in FIG. 1, an original document 22, suchas a sheet of paper, book, or the like, is placed face down upontransparent viewing platen 24. Lamp assembly 26, lens system 18 andfilter mechanism 20 are moved in a timed relation with drum 10 to scansuccessive longitudinally extending incremental areas of originaldocument 22 disposed upon platen 24. In this way a flowing light imageof original document 22 is projected onto photoconductive surface 12.During exposure, filter mechanism 20 interposes selected color filtersin the optical light path of lens 18. The filter operates on the lightrays passing through the lens to record an electrostatic latent image onphotoconductive surface 12 corresponding to a preselected spectralregion of the electromagnetic wave spectrum, hereinafter referred to asa single color electrostatic latent image. A suitable moving lens systemis disclosed in U. S. Pat. No. 3,062,108 issued to Mayo in 1962, and asuitable color filter mechanism is described in co-pending'applicationSer. No. 830,282 filed in 1969.

Drum 10 next rotates the single color electrostatic latent imagerecorded on photoconductive surface 12 to development station C.Development station C includes three individual developer unitsindicated by the reference numerals 28, 30 and 32, respectively. Thedeveloper units are all of the type generally referred to in the art asmagnetic brush developer units. Generally, a magnetic brush developerunit includes a magnetizable developer mix having carrier granules andtoner particles therein. The developer mix is continually broughtthrough a directional flux field to form a brush of developer mix. Thedeveloper mix is continually moving so as to provide the brush withconsistently fresh material. Development is achieved by bringing thebrush'of developer mix into contact with the electrostatic latent imagerecorded on photoconductive surface 12. Each of the developer units 28,30, and 32, respectively, includes toner particles corresponding to thecomplement of the specific color separated latent electrostatic imagerecorded on photoconductive surface 12. The respective toner particlesare selected to absorb light within a preselected spectral region of theelectromagnetic wave spectrum corresponding to the complement of thewave length of light transmitted through filter 20. Thus, a greenfiltered electrostatic latent image is rendered visible by depositinggreen absorbing magenta toner particles thereon, blue and red latentimages being developed with yellow and cyan toner particles,respectively. A suitable development station employing a plurality ofdeveloper'units is disclosed in co-pending application Ser. No. 255,259,filed in 1972.

In accordance with the present invention, additional toner particles areadded to the respective developer mix when the concentration thereof isbeneath a predetermined level. The regulating apparatus, indicatedgenerally at 34, includes reflecting means 36 mounted on photoconductivesurface 12 of drum 10. Beam generating means or light source 38 ismounted in housing 40 and arranged to direct a beam of light rays ontoreflecting means 36. Housing 40 includes a sidewall 60 having a shuttertherein associated with light source 38. Hence, the light rays fromlight source 38 are received by reflecting means 36 and prevented frombeing transmitted to photoconductive surface. An air purge systemcooperates with housing 40 to maintain the surrounding environmentsubstantially free from particle contamination. During development,toner particles are deposited on reflecting means 36 and the intensityof the light rays reflected therefrom is indicative of the densitythereof. Detecting means, such as photosensor 42, is adapted to receivethe light rays reflected from reflecting means 36. In response to thelight rays received. thereon, photosensor 42 produces an electricaloutput signal corresponding to the density of toner particles adheringto reflecting means 36. Comparing means, i.e., suitable logic circuitry,compare the electrical output signal from photosensor 42 with apredetermined reference signal. A logic control signal corresponding tothe deviation therebetween is generated for actuating the appropriatetoner particle storage container to dispense toner particles into thecorresponding developer unit. The detailed structural configuration ofregulating apparatus 34 will be described hereinafter in greater detailwith reference to FIGS. 2

and 3.

Continuing now with the description of the electrophotographic printingmachine processes, after development, drum 10 rotates to transferstation D. At transfer station D, the toner powder image adheringelectrostatically to photoconductive surface 12 is transferred to asheet of final support material 44. Final support material 44 may be,amongst others, plain paper or a sheet of thermoplastic material. Atransfer roll, shown generally at 46, recirculates support material 44in the direction of arrow 48. Transfer roll 46 is biased electrically toa potential of sufficient magnitude and polarity to attractelectrostatically toner particles from the latent image recorded onphotoconductive surface 12 to support material 44. A suitableelectrically biased transfer roll is described in U. S. Pat. No.3,612,677 issued to Langdon et al. in 1971. Transfer roll 46 rotates inthe direction of arrow 48 in synchronism with drum 10 (in this case atthe same angular velocity therewith). Inasmuch as support material 44 issecured releasably thereon for movement in a recirculating paththerewith, successive toner powder images may be transferred thereto insuperimposed registration with one another. Preferably, transfer roll 46includes a suitable recess therein for preventing reflecting means 36from engaging transfer roll 46. Thus, toner particles deposited onreflecting means 36 remain undisturbed by the transfer process andrepresent an indication of the toner particle concentration within thedeveloper mix.

After the toner powder images have been transferred to support material44, support material 44-is stripped from transfer roll 46 and advancedto a fixing station (not shown) where a suitable fuser coalesces thetransferred powder images thereto. One type of suitable fuser isdescribed in U. S. Pat. No. 3,498,592 issued to Moser et a]. in 1970.After the fixing process, support material 44 is advanced by a pluralityof endless belt conveyors (not shown) to a catch tray (not shown) forsubsequent removal therefrom by the machine operator.

Although a preponderance of toner particles are transferred to supportmaterial 44, invariably some residual toner particles remain onphotoconductive surface 12 after the transfer of the toner powder imageto support material 44. These residual toner particles are removed fromphotoconductive surface 12 as it passes through cleaning station E.Initially, the residual toner particles are brought under the influenceof a cleaning corona generating device (not shown) adapted to neutralizethe electrostatic charge remaining thereon. The neutralized tonerparticles are then mechanically cleaned from photoconductive surface 12by rotatably mounted fibrous brush 50. Rotatably mounted fibrous brush50 is positioned in contact with photoconductive surface 12 of rotatingdrum and reflecting means 36 so as to remove residual toner particlesremaining thereon after the transfer operation.

It is believed that the foregoing description is sufficient for purposesof the present application to illustrate the general operation of anelectrophotographic printing machine embodying the teachings of thepresent invention.

Referring now to the specific subject matter of the present invention,FIG. 2 illustrates the detailed construction of regulating apparatus 34.Regulating apparatus 34 includes reflecting means 36, light source 38,photosensor 42, and comparing means 37 having suitable logic circuitryassociated therewith for processing the electrical output signal. Inaddition, each of the developer units 28, 30, 32, respectively, havedispensing means 39 having a corresponding toner storage container anoscillator motor associated therewith. Each of the toner storagecontainers house a supply of selected toner particles having discretecolors so as to form a reservoir thereof for the appropriate developerunit. By way of example, the toner storage container of developer unit28 houses cyan toner, that of developer unit 30 magenta toner, and thatof developer unit 32 yellow toner. Each of the toner storage containersinclude perforations therein adapted to meter therefrom a specifledquantity of the selected toner particles to the corresponding developerunit. A suitable oscillator motor vibrates the appropriate toner storagecontainer to dispense toner particles. The toner particles pass throughthe perforations in the container to the corresponding developer unit.Regulating apparatus 34 actuates the oscillator motor to control thedispensing of toner particles from each of the toner storage containersto the respective developer unit..

As shown in FIG. 2, light source 38 produces a beam of light rays whichare reflected from reflecting means 36 to photosensor 42. Reflectingmeans 36 is located on a non-image portion of photoconductive surface12. As reflecting means 36 passes through the development zone,transparent electrically conductive layer 52 is biased to an electricalpotential simulating the electrostatic latent image recorded onphotoconductive surface 12. Preferably, electrically conductive layer 52is biased to about 200 volts above the developer bias, the normaldeveloper bias being about 500 volts. However,

electrically conductive layer 52 may be biased from about volts to about600 volts above the developer bias. The density of the image developedon electrically conductive layer 52 is sensed by photosensor 42. Theoutput signal from photosensor 42 is processed by suitable logicelements. Thereupon, depending upon the density of toner particlesdeposited on electrically conductive layer 52, toner particles may ormay not be added to the respective developer unit. Photosensor 42 ismounted exterior to and spaced from photoconductive surface 12 of drum10. It is positioned to sense the density of toner particles depositedon electrically conductive layer 52 just prior to the entering thereofinto cleaning station E. In this manner, electrically conductive layer52 undergoes the regular photoconductive drum cleaning process.

In order to apply the appropriate voltage corresponding to theelectrostatic latent image deposited on photoconductive surface 12,electrically conductive layer 52 must be biased to a suitable voltagelevel. This is achieved, preferably, by mounting a commutator assembly(not shown) in a region of the end bell of drum 10. A suitable slip ringassembly may be used in lieu of the commutator assembly. Timing for theapplication of the bias voltage to electrically conductive layer 52 maybe controlled by alternating means such as suitable electronic switchingor by use of split commutator ring, i.e. electrically conductive layer52 being biased over one portion of the commutator and not over theremaining portion thereof. The bias voltage is removed from electricallyconductive layer 52 during the cleaning process. It will be readilyapparent to one skilled in the art that instead of applying a biasvoltage to electrically conductive layer 52, a suitable bias may beapplied thereto by electrical charging.

By way of example, light source 38 is, preferably, a de-rated tungstenlamp with a regulated voltage, e.g. a 7 volt tungsten filament lampoperating from a 5 volt source. Photosensor 42 is a commerciallyavailable silicone phototransistor such as is produced by the GeneralElectric Company, Model No. L 14 B. Photosensor 42 is maintained in acontrolled thermal environment to minimize the effects of temperaturevariations. Preferably, oven 54 (FIG. 1) maintains the thermalenvironment of photosensor 42 at about 50 C. plus or minus 1 C., or, atany appropriate temperature suitable therefor.

Suitable logic circuitry processes the electrical output signal fromphotosensor 42. The logic circuitry, preferably, includes a suitablediscriminator circuit for comparing a reference with the electricaloutput signal from photosensor 42. The discriminator circuit may utilizea silicone control switch which turns on and effectively locks in afteran electrical output signal having a magnitude greater than thereference level is obtained. The signal from the discriminator circuitchanges the state of a flip-flop to develop an output signal therefrom.The output signal from the flip-flop, in conjunction with an outputsignal from the appropriate developer unit actuates an AND gate which,in turn, transmits a control signal to the oscillator motor of the tonerstorage container housing the toner particles corresponding to thedeveloper unit generating the output signal to the AND gate. The controlsignal also resets the flip-flop. The type of logic circuitry heretoforedisclosed is on-off. However, one skilled in the art will readilyrealize that it is possible to utilize proportional circuitry whichvaries the quantity of toner particles metered to the respectivedeveloper units as a function of the magnitude of the control signal.This may be readily achieved by a suitable integrated circuit moduleadapted to develop a stepped proportional dispensing rate. Duplicatelogic channels are utilized for each developer unit, i.e., the yellowdeveloper unit, cyan developer unit and magenta developer unit allutilize the foregoing logic circuits. Hence, there are three separate,independent logic channels, each channel being associated with itsrespective developer unit. The density of toner particles deposited onphotoconductive surface 12 is dependent upon the concentration of tonerparticles within the respective developer unit. The concentration oftoner particles is, in turn, a function of the magnitude of thereference signal. In this way, the image density of each color may beadjusted independently relative to one another. Thus, by adjusting therespective references image density as well as color balance isregulated within the multi-color electrophotographic printing machinedepicted in FIG. 1.

Turning now to FIG. 3, there is shown a perspective view of reflectingmeans 36. As shown therein, reflecting means 36 includes a transparentmember which may be a suitable glass slug 56. Adhering to one surface ofglass slug 56 is a reflective surface or silvered coating 58 adapted toreflect the light rays impacting thereon therefrom. Transparentelectrically conductive surface 52 is suitablysecured to the othersurface of glass slug 56. As shown in FIG. 3, electrically conductivesurface 52 includes two spaced portions, portion 52a and 52b.Preferably, portion 52a is adapted to transmit light rays from lightsource 38 therethrough to reflective surface 58. Reflective surface 58is arranged to reflect the light rays transmitted thereto throughportion 52b to impinge photosensor 42. Electrically conductive surface52 is made preferably from a transparent tin oxide coating. Atransparent electrically conductive glass, i.e. glass slug 56 andelectrically conductive surface 52, of this type is made by PittsburghPlate Glass under the trademark NESA, or is made by the Coming GlassCompany under the trademark Electro Conductive. As hereinbeforedescribed, reflecting means 36 is suitably mounted on drum l and isadapted to be electrically biased to the appropriate voltage level.

While the present invention has been described in connection withreflecting means 36 having a glass slug with one surface coated with areflective material and the other surface coated with a tin oxide layer,one skilled in the art will appreciate that the invention is notnecessarily so limited. For example, reflective means 36 may be apolished stainless steel disc. Preferably, the reflective disc would be09-1401 stainless steel having the reflecting surface polished tosubstantially about five rings flatness.

In recapitulation, a voltage is applied to reflecting means 36,preferably, simulating the electrostatic latent image recorded onphotoconductive surface 12. This voltage is automatically applied toreflecting means 36 by the orientation of the segmented commutator,hereinbefore discussed, relative to reflecting means 36 as drum 10rotates. Hence, just prior to rr: tering the development zone, a voltageof about 200 volts above developer bias is applied to reflecting means36. As drum 10 rotates in the development zone, the magnetic brushassembly of the respective developer unit applies toner particles toreflecting means 36 by the potential differential of approximately 200volts between reflecting means 36 and the respective developer unit. Asdrum l0 continues to rotate, light rays from light source 38 aretransmitted to reflecting means 36 and reflected therefrom tophotosensor 42. Photosensor 42, in turn, produces an electrical outputsignal which is compared by the logic circuitry with a reference todevelop a control signal for actuating the oscillator motor of theappropriate toner storage container housing the selected tonerparticles. In this way, toner particles are dispensed from the tonerstorage container into the developer mix of the respective developerunit. Thereafter, the biasing voltage is removed from reflecting means36, i.e. the split commutator ring rotates to an open circuit position,and is not biased thereafter. Cleaning station B then removes tonerparticles adhering to reflecting means 36. The aforementioned procedureis repeated three successive times for each copy produced, i.e. it isreproduced for each of the toner particles utilized within the printingmachine (cyan, magenta, and yellow). Hence, the regulating apparatus ofthe present invention controls the concentration of toner particleswithin the developer mix to create images having the requisite densityand color balance.

It is, therefore, apparent that there has been provided, in accordancewith this invention, an apparatus for regulating the concentration .oftoner particles within a development system that fully satisfies theobjects, aims and advantages set forth above. While this invention hasbeen described in conjunction with specific embodiments thereof, it isevident that many alternatives, modifications, and variations will beapparent to those skilled in the art. Accordingly, it is intended toembrace all alternatives, modifications and variations that fall withinthe spirit and broad scope of the appended claims.

What is claimed is:

I. An apparatus regulating toner powder concentration within 'aparticulate mix for a development system arranged to deposit tonerpowder on an image bearing member, including:

a light source;

a transparent member mounted on the image bearing member;

a reflective surface affixed to one surface of said transparent member;transparent electrically conductive surface affixed to the other surfaceof said transparent member, said conductive surface having at least twoportions spaced from one another with a portion of the exterior surfaceof said transparent member interposed therebetween, said light sourcebeing positioned so that the beam of light rays pass through one portionof said conductive surface onto said reflective surface for reflectiontherefrom through the other portion of said conductive surface;photosensor positioned so as to receive the light rays reflected fromsaid reflective surface and adapted to produce an electrical outputsignal corresponding to the intensity of light rays received thereon;means for comparing the electrical output signal with a reference toproduce a control signal corresponding to the deviation therebetween;and

means, energized by the control signal from said comparing means, fordispensing toner powder into the particulate mix of the developmentsystem.

2. An apparatus as recited in claim 1, further including means,operatively associated with said conductive surface, for cyclicallyalternating the electrical charge on said conductive surface to attracttoner powder thereto during the development thereof and to repel tonerpowder therefrom after the intensity of light rays reflected from saidreflective surface has been sensed by said photosensor.

3. An apparatus for regulating the developability of anelectrophotographic printing machine of the type having aphotoconductive member, and a development system utilizing a developermaterial having toner particles adapted to be deposited on anelectrostatic latent image recorded on the photoconductive member toproduce a toner powder image thereon, including:

a light source;

a transparent member mounted on the photoconductive member;

a reflective surface affixed to one surface of said transparent member;

a transparent electrically conductive surface affixed to the othersurface of said transparent member, said conductive surface having atleast two portions spaced from one another with a portion of theexterior surface of said transparent member interposed therebetween,said light source being positioned so that the beam oflight rays passthrough one portion of said conductive surface onto said reflectivesurface for reflection therefrom through the other portion of saidconductive surface;

a photosensor positioned so as to receive the light rays reflected fromsaid reflective surface and being adapted to produce an electricaloutput signal corresponding to the intensity of light rays receivedtherein;

means for comparing the electrical output signal with a reference toproduce a control signal corresponding to the deviation therebetween;and

means, energized by the control signal from said comparing means, fordispensing toner particles into the developer material of thedevelopment systern.

4. An apparatus as recited in claim 3, further including means,operatively associated with said conductive surface, for cyclicallyalternating the electrical charge on said conductive surface to attracttoner particles thereto during the development thereof and to repeltoner particles therefrom after the intensity of light rays reflectedfrom said reflective surface has been sensed by said photosensor.

1. An apparatus regulating toner powder concentration within aparticulate mix for a development system arranged to deposit tonerpowder on an image bearing member, including: a light source; atransparent member mounted on the image bearing member; a reflectivesurface affixed to one surface of said transparent member; a transparentelectrically conductive surface affixed to the other surface of saidtransparent member, said conductive surface having at least two portionsspaced from one another with a portion of the exterior surface of saidtransparent membeR interposed therebetween, said light source beingpositioned so that the beam of light rays pass through one portion ofsaid conductive surface onto said reflective surface for reflectiontherefrom through the other portion of said conductive surface; aphotosensor positioned so as to receive the light rays reflected fromsaid reflective surface and adapted to produce an electrical outputsignal corresponding to the intensity of light rays received thereon;means for comparing the electrical output signal with a reference toproduce a control signal corresponding to the deviation therebetween;and means, energized by the control signal from said comparing means,for dispensing toner powder into the particulate mix of the developmentsystem.
 2. An apparatus as recited in claim 1, further including means,operatively associated with said conductive surface, for cyclicallyalternating the electrical charge on said conductive surface to attracttoner powder thereto during the development thereof and to repel tonerpowder therefrom after the intensity of light rays reflected from saidreflective surface has been sensed by said photosensor.
 3. An apparatusfor regulating the developability of an electrophotographic printingmachine of the type having a photoconductive member, and a developmentsystem utilizing a developer material having toner particles adapted tobe deposited on an electrostatic latent image recorded on thephotoconductive member to produce a toner powder image thereon,including: a light source; a transparent member mounted on thephotoconductive member; a reflective surface affixed to one surface ofsaid transparent member; a transparent electrically conductive surfaceaffixed to the other surface of said transparent member, said conductivesurface having at least two portions spaced from one another with aportion of the exterior surface of said transparent member interposedtherebetween, said light source being positioned so that the beam oflight rays pass through one portion of said conductive surface onto saidreflective surface for reflection therefrom through the other portion ofsaid conductive surface; a photosensor positioned so as to receive thelight rays reflected from said reflective surface and being adapted toproduce an electrical output signal corresponding to the intensity oflight rays received therein; means for comparing the electrical outputsignal with a reference to produce a control signal corresponding to thedeviation therebetween; and means, energized by the control signal fromsaid comparing means, for dispensing toner particles into the developermaterial of the development system.
 4. An apparatus as recited in claim3, further including means, operatively associated with said conductivesurface, for cyclically alternating the electrical charge on saidconductive surface to attract toner particles thereto during thedevelopment thereof and to repel toner particles therefrom after theintensity of light rays reflected from said reflective surface has beensensed by said photosensor.